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Quiz 3: Pre/Postcondition Change

1. Which condition is weaker than $x > 0$ ?

$2x > 0$
$x \cdot x > 0$
$x + y > 0$
$x > 1$

Correct answer: $x \cdot x > 0$

Explanation:

Recall: $P_1 \iff P_2$ means the conditions are equivalent.

$x > 0 \iff 2x > 0$ , so the conditions are equivalent.

$x > 0 \implies x \cdot x > 0$ , so $x > 0$ is stronger.

$x > 0$ and $x + y > 0$ aren't comparable because $y$ isn't included in the implicant.

$x > 1 \implies x > 0$ , so $x > 1$ is stronger.

2. Which statement is correct?

$\text{length} = \text{count}$ is stronger than $\text{length} = (\text{count}-1)$
$\text{length} = \text{count}$ is neither stronger nor weaker than $\text{length} = (\text{count}-1)$
$\text{length} = \text{count}$ is weaker than $\text{length} = (\text{count}-1)$
$\text{length} = \text{count}$ is equivalent to $\text{length} = (\text{count}-1)$

Correct answer: $\text{length} = \text{count}$ is neither stronger nor weaker than $\text{length} = (\text{count}-1)$

Explanation:

It doesn't make sense to consider the implication relationship $\text{length} = \text{count} \implies \text{length} = (\text{count}-1)$ or vice-versa. Thus, there is no relationship between their contract strengths.

3. Which of the following is the most reasonable precondition of the method `deposit(double amount)` in the BankAccount class?

$\text{amount} \neq 0$
$\text{amount} \geq 0$
$\text{amount}$ is any double value
$\text{amount} > 0$

Correct answer: $\text{amount} > 0$

Explanation:

The action of "depositing" at a bank only makes sense when you're depositing a positive amount of money. This rules out all but $amount > 0$ .

4. In the following code, Module B results in a call of `sqrt(5)`. Which statement is correct?


// Module A
// Precondition: x >= 0
// Postcondition: return x's square root

double sqrt(double x) {
    ...
}


// Module B

double x = -5;
double y = sqrt(x);

assert abs(y * y-x) < eps

There is no bug in either A or B
There is a bug in both A and B
There is a bug in A
There is a bug in B

Correct answer: There is a bug in B

Explanation:

Recall the Pre/Postcondition Violation Rules.

Client B violates Supplier A's precondition, so the client (B) is at fault.